Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Artificial fluorescent or incandescent lighting is typically modulated by the mains frequency of the alternating current (AC) electrical power from the grid. Thus, in the US, lighting is typically modulated at 120 Hertz (due to 60 Hertz utility waveform reaching maximum voltage difference twice per cycle), and in Europe, lighting is typically modulated at 100 Hertz (due to the 50 Hertz utility waveform). Other light sources may be intensity modulated at other frequencies. For example, lighting on an aircraft is modulated by the 400 Hz AC power source used in aircraft. Light from a projection movie screen is modulated at about 24 Hz or 48 Hz, depending on the frame rate. Similarly, television and computer monitors emit light with a characteristic flicker that depends on the refresh rate of the display panel. Natural light environments (i.e., from the Sun) may thus be characterized by steady light—an absence of flicker modulation
Optical pulse meters operate by detecting optical signatures of pulsing blood flow through body tissue. A typical optical pulse meter includes a light source and a light sensor that is positioned to detect light emitted by the light source that is either reflected from the body tissue or transmitted through the body tissue. The optical signals provide indications of pulsing blood flow due to periodic variations in the amount of reflected/transmitted light due to the time-variant amount of blood in the vasculature structure. The intensity of measured light depends on the amount of blood in the body tissue (e.g., vasculature structure) during a given measurement. Periodic variations in the intensity of measured light are due to pulsing of the blood. Thus, the pulse rate can be determined based on the intensity modulation frequency of a set of light sensor measurements. In addition, the oxygenation level of the arterial blood can be determined based on the strength of the reflected/transmitted light signal. Hemoglobin-containing blood is more transmissive to red light than to infrared light. Thus, one example may involve illuminating body tissue with both red light and infrared light and using the difference in the light reflected/transmitted at the two wavelengths to isolate effects from pulsing arterial blood as opposed to other body tissue and/or venous blood. For instance, by observing differences in the relative amount of light reflected and/or transmitted at multiple wavelengths, such as red and infrared light ranges, the time-variant amount of blood can be evaluated, and the oxygenation level and/or pulse rate can be determined.